Re: st: Analysis of experiment involving baseline measurements

Phil Jones asks for advice in adjusting for baseline measurements when
analyzing data with two follow-up points.
You need to first think about what theory underlies the intervention and
the implications for how the outcome score will evolve over time--the
modeling will depend on that. Do you expect both groups to improve from
pre to post and continue to improve at 6wks? If so, will they continue to
improve at the same rate as from pre- to post-, or will there be a
tapering off (or an acceleration)? Or do you expect the scores to
deteriorate somewhat at 6 wks?
If you -reshape- your data into long format, you can model any of these
possibilities using -xtmixed- or -xtreg-. The independent variables
specification may involve a single degree-of-freedom specification of
time, or time as a factor variable, or perhaps as a spline. And your
representation of time will then have interaction terms with group. You
will also have the option of either including the baseline value as a
covariate (and not analyzing time = pre observations) or not. But you
have to have a model of the time-trajectories of the output in mind to
make the corresponding decisions.
Hope this helps you make progress.
Clyde Schechter
Department of Family & Social Medicine
Albert Einstein College of Medicine
Bronx, NY, USA
Please note new e-mail address: clyde.schechter@einstein.yu.edu
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